Slack adjuster



l`0V 29, 1938- o. RAsMUssEN ET AL 2,138,206 I sLAcx ADJusTER Filed May2e, 193e .fs fsf., @4 3mm /59 186 1%( j 9/af Wsnzwez /sf y l zal /51@1k/mid WM@ Girod/swap Patented Nov. 29, 1938 SLACK ADJUSTER OlafRasmussen and Richard C. Rike, Dayton, l Ohio, assignors to GeneralMotors Corporation, Detroit, Mich., a corporation of DelawareApplication May 28, 1936, Serial No. 82,207

lClaiilL This invention relates to brakes, and hasbeen designed toautomatically take up excessive clearances between the drum and shoes ofa hydraulically operated vehicle brake.

An object of the invention is to provide means to take up excessiveclearances by mechanism suitably housed within the so-called Wheelcylinder of the hydraulic brake system.

Another object is to provide such an adjustment superimposed upon 'apredetermined lost motion connection in the applying means designed toensure a necessary degree of shoe clearance.

Other objects comprise the accomplishmentof the major objects bymechanism which shall be simple, easy to install, efficient in operationand comparatively inexpensive.

The invention is illustrated on an accompany# ing drawing in which Fig.1 is a transverse sectionthrough a brake drum and wheel cylinder.

Figs. 2 and 3 are similar sections of modied forms.

Fig. 4 is a section online 4-4 of Fig. 3.

Fig. 5 is a similar section of ystill another embodiment.

Referring by. reference characters to the drawing, numeral 1 is used todesignate a conventional brake drum. To frictionally engage the drumthere are shown two shoes 9 and II normally held from drum contact by areleasing spring I3.

Suitably supported as by the backing plate or cover for drum 1 is awheel cylinder I5. Fluid is admitted through an opening I1 to an inter-A mediate part of the cylinder by means of a conduit such as I8 (seeFig. 4) from a master cylinder, not.shown. At I9 is an air bleed holewhich-is normally closed by a plug 20 (see Fig. 4).

Within the cylinders are pistons 2l and 23 provided with sealing cups25. Between the pistons there may be a spring 21.

A xial members or plungers 29 have bifurcated heads 3| embracing thewebs of shoes 9 and II.' Externally each plunger is provided with ahelical ratchet tooth formation 30. The plungers engage the pistons andthe spring I3 functions to push the plungers and pistons to brakerelease posi-- tion-the position shown. 'I'he cylinder adjacent each endis formed with a shoulder '33fengagedby a radially extended part of apawl retainer 35. The retainer is held against the shoulder 'by a vcover31 secured by screws or the like 39. A dust guard 4I preferably ofrubber is held to the cylinder adjacent an extension (ci. iss-19.5)

43 of the cover and grips the end of the shoe. Within the retainer andpartly positioned by the cover 31 are a plurality of radially directedpawls 45, their teeth engaging the ratchet device 30. A suitable spring32 serves to project the pawls into the position shown. It will be notedthat since the ratchet formation is helical and since the pawls larecircumierentially spaced they engage at slightly different positionsaxially of the plunger, this being permitted by the spaced l29. At thesame time pawl 45 lies against the cover 31 and is not holding theplunger from inward movement. The uid entering at I1 moves the piston 2I(and also 23) toward the end of the shoe. The shoe is moved to contactthe drum by means of plunger 29. If the lining is not worn appreciablythe movement of the plunger will cause the tooth of the lower pawl 45 tslide toward but not over the top of the cooperating ratchet tooth. Atthe same time the tooth of the other pawl remains at the bottom of theratchet as the pawl moves toward a position corresponding to that shownby 45. It is duringthis movement of pawl 45a from the position shown toa position like that of 45 in Fig. 1 that the shoe clearance necessaryto accommodate changes in the drum is taken up. 'Ihe lining wear may besuch that the tooth of pawl 45 jumps over a ratchet tooth. If so, uponthe return of the plunger, pawl 45l may move to the right suilicientlyto engage the wall of the retainer and hold the plunger. aiorded a iixeddimension lost motion to accommodate drum changes, and there is alsoprovided a liner adjustment than would be the case if circular teethwere cut on the plunger or if but a single pawl were used with a helicalgroove. Obviously more than two pawls may be used if still fineradjustmentbe needed. Still another advantage results from the use of ahelical groove. When the liningis replaced it is possible to re There isthus lease plunger29irom the shoe and, merely by 55 10 relation of thewalls of the retainer and cover,

rotating it, to relocate the pawls toward the bifurcated end of theplunger. It is not necessary to provide any means to lift the pawl aswould be the case with a circular groove.

By the location within the cylinder and by means of the protectionaiorded by the boot, the self-adjusting expedient is fully protectedfrom foreign matter and manual adjustment is not required throughout thelife of the lining.

In Fig. 2 is shown a somewhat modified form. In this embodiment the drumand shoe are designated by the same reference characters as are used inFig. 1. The same piston 2| slides in a slightly modified cylinder |5a.Into the cylinder |5a is inserted a retainer |35 to engage a shoulder|33. A disc |31 closes a chamber within the retainer and the assembly isheld in position by a nut |39'threaded into the end of the cylinder. Thepawls are marked |4| and are projected into engagement with the helicaltoothed periphery of a sleeve-like ratchet |43. The end -of the ratchetsleeve is bifurcated and embraces the web of the shoe. Reciprocablewithin ratchet sleeve |43 is a plunger |45 operably engaging the shoeweb and having a head |41 to contact with the piston 2|. It will benoted that when both` the ratchet `and the plunger engage the shoe webas they do when the brake is released, the length of the plunger is suchthat the head |41 is spaced slightly from the adjacent end |48 of theratchet sleeve. The pawls act in sequence in determining the releaseposition because of the helical character of the ratchet. The space |50between the head |41 and the ratchet sleeve is made sufficient toprovide a necessary clearance between the drum and shoes. Such aclearance -is required to accommodate drum changes such as may be causedby heat or distortion under the inuence of pressure. When the head 2| ismovedby the piston the opposite end pushes the shoe and spaces the shoefrom the ratchet sleeve. When the head engages and moves the ratchetsleeve the pawls may or may not determine a new retracted position ofthe shoe dependent upon the condition` of the lining. Since the end ofthe plunger has moved the shoe from contact with the ratchet sleeve, theplunger will return suillciently to develop a spaced relation betweenthe shoe and drum, equivalent to the illustrated space between the head2| and the ratchet sleeve, before the ratchet sleeve engages and stopsthe release movement of the shoe. Sufficient shoe clearance is thusensured to accommodate contraction of the drum as it cools andrestoration from any distortion which may have been imparted to it inthe act of brake application.

The number of the pawls determines the neness of the adjustment forcorrecting lining wear. 'I'his form of the invention is believed topossess especial merit in that there is a predetermined spacingbetweenthe plunger and ratchet sleeve to accommodate drum changes regardless ofthe variable position of the ratchet sleeve due to lining wear.

In Fig. 3 is a modification employing the same drum, shoes and piston.Cylinder 5b hasa pawl retainer |5| carrying a plurality of axiallyspaced pawls |53 projected by springs |55 into contact with circularratchet teeth |51 on a ratchet sleeve |59. A plate |6| and snap ring |63position the parts. The axial spacing of the pawls and the circular(instead of helical) ratchet teethare to secure the flne adjustment.Fig. 4 shows that there may be four of these pawls. Here, as before, thespacing of the head |41 of plunger |45 from the end |52 of the ratchetsleeve |59 is for the purpose of accommodating changes in drum contour,the pawl and ratchet functioning solely to accommodate lining wear.

In some hydraulic brake wheel cylinders there is used a piston sealsecured peripherally to the cylinder, in which cases it is not expedientthat the piston assume new positions of adjustment outward toward theend of the cylinder for' changes in lining wear. Fig. 5 shows anadaptation of our invention for a wheel cylinder having such a pistonseal. In this figure the drum 1 and shoe 9 are like those of the otherembodiments. V'I'he piston |65 is advanced by an expansible seal, seal|61 being representative of that type which is anchored to the cylinderinstead o! slidable with the piston. The seal is secured peripherally inthe junction of the central part |69 of the cylinder and an end partthereof marked |1|. Screw means |13 is shown as securing together parts|1| to |89 and its extended end |15 is received in a notch of the piston|85 to guide the latter. The piston is provided with pawls |11 projectedradially by springs |19 to engage a helical ratchet formation on aplunger |8|. A closure |83 secured by screws |85 positions the pawls.Carried by the plunger is a fixed ring |84 for a purpose to beexplained. A disc |86 is held against a shoulder near the end of part|1|. An end closure |81 is secured by fastening means |89. Between parts|86 and |81 are pawls 9| projected by springs |93 to engage a helicalratchet formation on the outer surface of ratchet sleeve |91. Sleeve |91is bifurcated to straddie and, in brake release position, to engage theweb of shoe 9 along with the head |99 of the plunger |8| which ismounted to reciprocate through the ratchet sleeve. It will be seen thatwhen both the bifurcated end of the ratchet sleeve |91 and the head 99of the plunger contact with the shoe web, the ring |84 is spacedslightly from the end of the ratchet sleeve. Between the disc |83 andthe part |86 is a coil spring 20|. The operation is as follows: Fluidadmitted behind the seal |81 bulges the seal to a convex form and thepiston |65 is advanced.

Since the pawls lock the plunger to the piston for this direction ofmovement, the plunger ,|8I `is 'advanced and the shoe is moved tocontact the drum. After a limited movement. ring |84 engages the end ofthe ratchet |91and if there has been an appreciable lining wear one ofthe pawls |9| may jump over the ratchet tooth and deteradded provisionbe made to this end. In the forms described above, the piston didassumenew positions. In those cases, however, the seal moves with thepiston and since the reservoir could be relied on to supply theadditional fluid required by the new position of the piston, nodisadvantage results. When, however, the seal is carried by thecylinder, the piston should return to the same initial position aftereach brakev application. In the modication shown by Fig. 5, this canoccur after the brake has been relea'sed. Spring 20| acts to push thepiston |85 back to its original position, the pawl |11 merely traversingthe ratchet teeth of the plunger. The pawl is then Vavailable to enablethe piston at the beginning o! its movement to initiate the movement ofthe plunger. It will be understood that this expedient may be used witha conventional seal as in the other gures. but it is of especial use 5with such seals as those typiiled by diaphragm 10 retard its rotation,brake applying means com prlsing a cylinder, a piston element therein,mo-

tion transmitting means between said piston element and shoe, saidtransmitting means including a reciprocable member having a plurality ofaxially displaced circumferentially disposed teeth,

a plurality of pawls carried by the cylinder and .spacedcircumferentially, said pawls being disposed axially relative to eachother, and yielding means pressing said pawls radially toward saidteeth.

,OLAF RASMUSSEN. RICHARD C. BIKE.

